Researcher shows that the weak cosmic principle holds even for extreme black …

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Einstein's general theory of relativity has always presented some problems for physicists. On the plus side, it predicts the behavior of mass, space, and time with astonishing accuracy. On the downside, the mathematics produces singularities. That is, there are some situations in which the only valid answer is infinity and physicists really, really hate that. The infinities in Einstein's theory of gravity are one of the things that tell us that there is probably a more complete theory available, and we just need the intelligence to go out and find it.

In the meantime, Penrose and company have come up with what is called the weak cosmic censorship principle, which tells us that all singularities are hidden from us by black holes. What does this actually mean? It means that we don't know what space-time at a singularity actually looks like, but that doesn't matter because each singularity is surrounded by an event horizon, beyond which we cannot observe.

Now the nice thing about black holes is that they are relatively simple objects, which means that, even in the absence of observations, we can make a lot of predictions about their behavior. One of these predictions describes the conditions under which an event horizon remains intact. It turns out that if you add enough charge or angular momentum (spin) to a black hole, its event horizon could be cast off. This would expose the singularity for all to see, leaving the universe very embarrassed (possibly sending it into rehab).

So the big question is how space-time responds when we start dropping massive amounts of charge or rotation into a black hole. Early calculations indicated that the event horizon could indeed vanish, but these calculations were quite simple, assuming conditions where the black hole sat in a passive space-time and the charge or spin was simply dropped in. In reality, space-time is influenced by both the black hole and the "beams" of angular momentum. Shahar Hod from the Hadassah Institute in Israel has now calculated how all these interact.

The first thing to note is that a black hole will only absorb certain chunks of angular momentum, and this behavior depends on how much it is already spinning. When the two momentums don't match, the beams just bounce off. Second of all, the space-time surrounding the black hole will rotate as well. When the two are taken into account, we find that, even if we sit a black hole right on the edge of exposing its singularity and add spin, the black hole can never quite shake the event horizon free, because the surrounding space-time compensates. Similarly, when one drops charge on a black hole, the interaction between the black hole, the charge, and space-time causes the black hole to internally rearrange so that it presents a mirror charge. This resulting repelling force becomes stronger and stronger as the black hole gets nearer and nearer to the threshold of exposing itself, meaning that charge begins to bounce off.

So, for all of you out there worried about being exposed to the dirty side of the universe: fear not, the cosmic censor is stronger than it first appeared.

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He Lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com